1. Field of the Invention
The present invention relates to broad bandwidth trace detection apparatus and techniques. More specifically, the present invention relates to highly sensitive real-time spectroscopy.
2. Problems in the Art
Over the past century enormous effort has been invested in the development of spectroscopic methods for monitoring and making quantitative measurements of the physical world. As a result modern, spectroscopic approaches represent some of the most precise and widely used measurement tools. Frequency measurements such as the 1 S to 2 S transition in hydrogen (1) are rapidly improving, providing ever more rigorous tests of fundamental theories and creating insights for more in-depth investigations of atomic structure. Similarly, new and more powerful spectroscopic techniques are continually in demand for challenging chemical physics applications such as recording the high overtone spectrum of H3+ (2) or observing the isomerization process in vinylidene-acetylene (3). Spectroscopy capable of such measurements is essential for the verification and further development of molecular theory. More practical applications of spectroscopy such as the real-time detection of trace amounts of molecular species are in demand in varying contexts. Such applications range from a security staging area in an airport for detection of trace amounts of molecules found in explosives or biologically hazardous materials to a doctor's office where a patient's breath could be analyzed as a non-intrusive method for monitoring diseases such as renal failure (4) and cystic fibrosis (5). Spectroscopic systems capable of making the next generation of atomic and molecular measurements will require: i) A large spectral bandwidth allowing for the observation of global energy level structure of many different atomic and molecular species; ii) High spectral resolution for the identification and quantitative analysis of individual spectral features; iii) High sensitivity for detection of trace amounts of atoms or molecules and for recovery of weak spectral features; iv) A fast spectral acquisition time, which takes advantage of high sensitivity, for the observation of spectral changes due to changing environmental conditions, leading to the study of dynamics.
Unfortunately, the characteristics of a good spectroscopic system are often in competition with each other. For example, designing a system with a large spectral bandwidth and high resolution (or high sensitivity) is fundamentally challenging due to the difficulty of selecting a narrow spectral band from a broad-spectrum source. As a result of such trade-offs, modern spectroscopic methods which are designed to meet two or three of the desired system characteristics with excellent performance will function poorly in the remaining areas. Single pass absorption techniques such as Fourier transform infrared (FTIR) (6) and wavelength agile methods (7) do an excellent job of providing large bandwidths up to several hundreds of nanometers and achieve remarkably fast acquisition times by recording entire spectra in microseconds. However, these methods offer sensitivities that are many orders of magnitude too low for applications involving trace detection or observation of weak spectral features. Both of these techniques are capable of achieving high resolution, but at a cost. For FTIR, the cost of high resolution is prolonged acquisition times. For wavelength agile techniques, high resolution is attained only if the spectral bandwidth is drastically decreased. Contrarily, cavity enhanced techniques such as noise immune cavity enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) (8), and cavity ringdown spectroscopy (CRDS) (9) offer incredibly high sensitivities of 1 part in 1010 and beyond at 1 s averaging time and can provide high resolution, but these methods are generally limited to small spectral bandwidths of a few nanometers. Newer approaches to cavity enhanced spectroscopy have been directed at increasing the spectral bandwidth and reducing the acquisition time (10, 11, 12, 13). Such efforts have demonstrated large bandwidths of up to 50 nm with an acquisition time of 2 s (12), and fast acquisition times of 1 ms for a bandwidth of 0.5 nm (13). However, these methods have yet to demonstrate tens of nanometers of spectral bandwidth at millisecond acquisition times.
There remains a need in the art for spectroscopic methods and apparatus that address all of the mentioned system characteristics, yielding a powerful combination of bandwidth, sensitivity, resolution, and acquisition speed unmatched by any existing approaches.